Electrochemical CO2 Reduction by Polymer-Encapsulated Cobalt Phthalocyanine: Controlling Coordination Environment to Modulate the Activity and Selectivity
Liu, Yingshuo
2020
Abstract
The discovery of systems that preferentially convert carbon dioxide to single products while suppressing competitive side reactions like hydrogen evolution is a critically important outstanding challenge in renewable energy storage and carbon dioxide remediation. The research approach examined in this thesis is to encapsulate molecular catalysts within hydrophobic coordinating polymers that influence the primary-, secondary-, and outer-coordination spheres of the catalytic active site and also control substrate delivery to promote selective CO2 reduction (CO2RR) and inhibit competitive side reactions. Toward this goal, cobalt phthalocyanine (CoPc) encapsulated in poly-4-vinylpyridine (P4VP) has been adsorbed to the carbon electrodes by physisorption to the carbon surface. The mechanism of CO2RR by polymer-encapsulated CoPc has been investigated. Specifically, a series of electrochemical kinetic isotope effect (KIE) measurements and proton inventories on CoPc-P4VP and related systems were conducted to probe the mechanistic implications of primary-coordination and the proton involvement in the rate-determining step of the CO2RR mechanism. These studies provide strong evidence that coordination of an axial ligand changes the rate-determining step of CO2RR. Moreover, it has been confirmed that there exists proton relays in the CoPc-P4VP system. This work highlights the importance of both primary- and outer-coordination sphere effects in the rate and selectivity of CO2RR by catalyst-polymer composite systems. In collaboration with the Penner-Hahn group at the University of Michigan, in situ X-ray absorption near edge structure (XANES) was used to verify that Co is four-coordinate in CoPc, five-coordinate in axially coordinated CoPc(py), and mostly, but not completely, five-coordinate in CoPc-P4VP. In addition, the coordination environment of CoPc-P4VP is pH-dependent, suggesting that the axial coordination of pyridyl groups in P4VP with CoPc is modulated by the protonation of the polymer membrane. Interestingly, different oxidation state changes for four- and five-coordinate CoPc upon reduction suggest that the reduced five-coordinate species have a HOMO with metal character which is different than the four-coordinate CoPc species and this may partially explain the increased activity for CO2RR for the five-coordinate species. Finally, in order to probe the effect of axial coordination on CoPc, a series of CoPc(L) complexes where the σ-donor strength of L is varied was examined. There is an increase in the observed overall electrochemical activity of the corresponding CoPc(L) as L moves from less to more electron donating strength as indicated by density functional theory (DFT) calculations. This result suggests that the increased CO2RR activity observed upon axial coordination to CoPc is due to the increased energy of the dz2 orbital, which is crucial for the development of new electrocatalysts for CO2RR.Subjects
Electrochemical CO2 Reduction Polymer Encapsulation
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